Having established how to calculate the costs for concrete mixes and mixers, we have now to determine the cost oflabour required to produce, transport and place the concrete mix.
As the essence ofgood concrete work is that continuous production be maintained, the type ofjob to be done and the mixer being used determines the number ofmen in a gang or squad.
We could adopt labour constants; that is, average outputs for each type of concrete component, which would account for producing, transporting and placing a mix. However, a more accurate way to develop the costs is to determine the output ofa mixer in m3 per hour and to estimate the gang size required to mix, transport and place. The gang cost per m3 is then calculated by dividing the gang cost per hour by the mixer output in m3 per hour.
The rate ofdischarge is determined by the demand on the mixer on site and by its capacity in terms ofnumber ofbatches per hour.
Mixer outputs and gang sizes
Figure 6.1 shows mixer outputs and related gang sizes. The gangs given on the graph are for mixing only and would be made up as follows:
One-man gang Mixer operator
Two-man gang Mixer operator, feeder operator
Three-man gang Mixer operator, feeder operator and labourer.
It has been assumed that mechanically fed mixers are used, i.e. those with a feed hopper and dragline aggregate feeder. Where a mixer is fed manually, another labourer should be added to the squad.
Under the Working Rules agreement WR3, extra payments are made for extra skill or responsibility. Operatives responsible for a mixer (one per mixer) and for a dragline feeder are entitled to such extra payments, which are given in the Working Rules.
Placing outputs
Placing outputs are dependent on the type ofconcrete member to be poured and the mixer output. The gang size will therefore depend on these two factors. Table 6.2 indicates required gang sizes for a selection of typical members.
Transporting outputs
Concrete is normally transported from its point of mixing to the required position on site by ordinary wheelbarrow or diesel powered dumper. On the average building site, where the terrain is rough and messy, a wheelbarrow is uneconomical for transporting concrete beyond about 60 m. A dumper can be used on distances ofup to 400 m. However, the aggregates in a concrete mix can be settled out by the action ofa dumper, and therefore it is better to minimize journey lengths.
A labourer transporting concrete using a wheelbarrow will do so at about 1 m3/hour over a 100 m round trip.
1 2 3 4 5 6 7 8 9 10 11
5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0
2 man gang
3 man gang
1 man gang
Mixer size 21/12
18/12
14/10
10/7
3 Mixer output (m/hour) 7/5
Rate of discharge (min/mix)
Figure 6.1 Mixer outputs and related gang sizes
A standard dumper of368 litres working at 20 per cent below maximum capacity can take full discharge from mixers up to 14/10 capacity. For larger mixers, a swivel skip dumper of say 510 litres would need to be used. Dumper costs can be calculated using the method adopted earlier for mixer costs. An average speed of 30 km/hour can be used in the calculation ofdumper transport costs.
Other plant may be needed, such as a hoist to take a concrete mix from ground level to floor levels above. If this is the case then costs for such plant would have to be assessed and included in the rate build-up.
Example 6.3
Plain in situ concrete in foundations; grade C25P, 20 mm aggregate, medium workability; poured on or against earth or blinded hardcore;
not exceeding 300 mm thick. m3
We can begin by taking the materials cost ofthe mix direct from Example 6.2 Materials
Cost per m3 £72.66
Waste: add 5% £3.63
Mixer
We will use a 10/7 concrete mixer, and assume that the cost for this mixer is £1.06 per hour. From the mixer output graph (Figure 6.1) we can see that a 10/7 mixer discharging at say 3 minutes per mix will produce approximately 4 m3/hour.
Therefore
Mixer cost per m3: £1.06/4 £0.27
Mixing squad
From Figure 6.1 we see that the gang size required is 3 men. Therefore Table 6.2 Gang sizes required to pour particular members
Mixer output (m3/hour)
Members and description 0±4 4±8 8±12
Foundations in trenches, over 300 mm thick 1 1 2
Foundations in trenches, not exceeding 300 mm thick 1 2 2
Beds not exceeding 150 mm thick 2 2 3
Beds 150±450 mm thick 1 2 2
Beds over 450 mm thick 1 1 2
Walls not exceeding 150 mm thick 2 3 3
Walls 150±450 mm thick 2 2 3
Walls over 450 mm thick 2 2 3
Columns 2 2 3
Staircases 2 3 3
Mixer operator cost/hour £6.73
Feeder operator cost/hour £6.77
Labourer cost/hour £6.50
Gang cost/hour £20.00
Gang cost per m3: £20.00/4 £5.00
Transporting
Transporting mix by wheelbarrow over say a 50 m round trip, assuming that 1 labourer will transport 1 m3/hour over 100 m:
Labourers (nr) to barrow 4 m3/hour: (50/100) 4 2 Cost to transport 4 m3: 2 £6:50 £13:00
Cost to transport 1 m3: £13.00/4 £3.25
Placing
From Table 6.2 we see that 2 labourers are required to spread the mix. Therefore
Cost per m3for 1 labourer at £6.50: £6.50/4 £1.63
£86.44
Profit and oncost 20% £17.29
Rate per m3 £103.73
Example 6.4
Plain in situ concrete in beds; grade C25P, 20 mm aggregate, medium workability; poured on or against earth or blinded hardcore;
150±450 mm thick. m3
Again we can take the materials cost ofthe mix direct from Example 6.2 Materials
Cost per m3 £72.66
Waste: add 5% £3.63
Mixer
We will use a 14/10 concrete mixer, and assume that the cost for this mixer is £2.38 per hour. From the mixer output graph (Figure 6.1) we can see that a 14/10 mixer discharging at say 3 minutes per mix will produce approximately 5.75 m3/hour.
Therefore
Mixer cost per m3: £2.38/5.75 £0.41
Mixing squad
From Figure 6.1 we see that the gang size required is 3 men. Therefore
Mixer operator cost/hour £6.73
Feeder operator cost/hour £6.77
Labourer cost/hour £6.50
Gang cost/hour £20.00
Gang cost per m3: £20.00/5.75 £3.48 Transporting
Transporting mix by wheelbarrow over say a 100 m round trip, assuming that 1 labourer will transport 1 m3/hour over 100 m:
Labourers (nr) to barrow 5.75 m3/hour: (100/100) 5.75 5.75 (say 6) Cost to transport 4 m3: 6 £6:50 £39:00
Cost to transport 1 m3: £39.00/6 £6.50
Placing
From Table 6.2 we see that 2 labourers are required to spread the mix. Therefore
Cost per m3for 2 labourer at £6.50: 2 £6.50/5.75 £2.26
£88.94
Profit and oncost 20% £17.79
Rate per m3 £106.73
Example 6.5
Reinforced concrete walls; grade C25P, 20 mm aggregate, medium
workability; not exceeding 150 mm thick. m3
The following costs per m3are taken from Example 6.4:
Materials £72.66
Waste: add 5% £3.63
Mixer £0.41
Mixing squad £3.48
Transporting £6.50
Placing costs are now added. From Table 6.2 we see that 3 labourers are required to spread the mix. Therefore
Cost per m3for 3 labourers at £6.50: 3 £6:50/5:75 £3.39
£90.07
Profit and oncost 20% £18.01
Rate per m3 £108.08